1. Field
The present application relates to an article for magnetic heat exchange, in particular an article for use as a working medium in a magnetic heat exchanger, and methods of fabricating an article for magnetic heat exchange.
2. Description of Related Art
Magnetic heat exchangers include a magnetocalorically active material as the working medium to provide cooling and/or heating. A magnetocalorically active material exhibits the magnetocaloric effect. The magnetocaloric effect describes the adiabatic conversion of a magnetically induced entropy change to the evolution or absorption of heat. By applying a magnetic field to a magnetocalorically active material, an entropy change can be induced which results in the evolution or absorption of heat. This effect can be harnessed to provide refrigeration and/or heating.
The magnetic entropy of the material changes depending on whether a magnetic field is applied or not owing to the difference between the degrees in freedom of the electron spin system. With this entropy change, entropy transfers between the electron spin system and the lattice system.
A magnetocalorically active phase, therefore, has a magnetic phase transition temperature Ttrans at which this phase change occurs. In practice, this magnetic phase transition temperature translates as the working temperature. Therefore, in order to provide cooling over a wider temperature range, the magnetic heat exchanger requires magnetocalorically active material having several different magnetic phase transition temperatures.
A variety of magnetocalorically active phases are known which have magnetic phase transition temperatures in a range suitable for providing domestic and commercial air conditioning and refrigeration. One such magnetocalorically active material, disclosed for example in U.S. Pat. No. 7,063,754, has a NaZn13-type crystal structure and may be represented by the general formula La(Fe1-x-yTyMx)13Hz, where M is at least one element of the group consisting of Si and Al, and T may be one of more of transition metal elements such as Co, Ni, Mn and Cr. The magnetic phase transition temperature may be adjusted by adjusting the composition.
In addition to a plurality of magnetic phase transition temperatures, a practical working medium should also have a large entropy change in order to provide efficient heating. However, elemental substitutions which lead to a change in the magnetic phase transition temperature can also lead to a reduction in the entropy change observed.